The project is designed to investigate the metabolism of cell nuclei in the retina of the mouse. Work in progress may be summarized in three categories of studies: (1) Analysis of the duration of different components of the cell division cycle during the period when cell division is coming to a close during differentiation of the retina; (2) Analysis of the ultrastructural changes in retinal nuclei during this same period, and during the interval when neuroblasts are converted to fully differentiated post-mitotic cells. Emphasis is being placed on the differentiation of nuclei in rods and in bipolar cells; (3) Analysis of the chemical constituents of retinal nuclei from mature mice, using biochemical procedures. Studies soon to be initiated involve analysis of histone renewal and DNA repair.